The port buffer congestion threshold provides an early
warning to the switch that the port is about to
exhaust all its guaranteed buffers, leading to congestion.
This threshold is expressed as a percentage of the port's
total guaranteed buffer depth. Once this threshold has
been crossed, the switch begins colliding with frames
received on the port for a fixed period of time.
After this time has expired, the switch determines
whether the port's buffer congestion has been alleviated.
If this situation has not changed, the switch resumes
this congestion control algorithm on the port.